SystemClock

Three different clocks are available, and they should not be confused:

System.currentTimeMillis()
is the standard "wall" clock (time and date) expressing milliseconds
since the epoch. The wall clock can be set by the user or the phone
network (see setCurrentTimeMillis(long)), so the time may jump
backwards or forwards unpredictably. This clock should only be used
when correspondence with real-world dates and times is important, such
as in a calendar or alarm clock application. Interval or elapsed
time measurements should use a different clock. If you are using
System.currentTimeMillis(), consider listening to the
ACTION_TIME_TICK,
ACTION_TIME_CHANGED
and ACTION_TIMEZONE_CHANGEDIntent
broadcasts to find out when the time changes.

uptimeMillis() is counted in milliseconds since the
system was booted. This clock stops when the system enters deep
sleep (CPU off, display dark, device waiting for external input),
but is not affected by clock scaling, idle, or other power saving
mechanisms. This is the basis for most interval timing
such as Thread.sleep(millls),
Object.wait(millis), and
System.nanoTime(). This clock is guaranteed
to be monotonic, and is suitable for interval timing when the
interval does not span device sleep. Most methods that accept a
timestamp value currently expect the uptimeMillis() clock.

elapsedRealtime() and elapsedRealtimeNanos()
return the time since the system was booted, and include deep sleep.
This clock is guaranteed to be monotonic, and continues to tick even
when the CPU is in power saving modes, so is the recommend basis
for general purpose interval timing.

The Handler class can schedule asynchronous
callbacks at an absolute or relative time. Handler objects also use the
uptimeMillis() clock, and require an event loop (normally present in any GUI application).

The AlarmManager can trigger one-time or
recurring events which occur even when the device is in deep sleep
or your application is not running. Events may be scheduled with your
choice of currentTimeMillis() (RTC) or
elapsedRealtime() (ELAPSED_REALTIME), and cause an
Intent broadcast when they occur.

Causes the current thread to wait until another thread invokes the
notify() method or the
notifyAll() method for this object, or
some other thread interrupts the current thread, or a certain
amount of real time has elapsed.

sleep

Waits a given number of milliseconds (of uptimeMillis) before returning.
Similar to sleep(long), but does not throw
InterruptedException; interrupt() events are
deferred until the next interruptible operation. Does not return until
at least the specified number of milliseconds has elapsed.